Human replication protein A ('1RPA) is an essential single-stranded DNA-binding protein which stimulates the activities of DNA replication and repair proteins by physical interaction with them. It exists as a stable heterotrimer of 70, 29, and 11 kD subunits. There are at least two binding modes for ~A and ssDNA, IlRPA:ni and ~ The STEM was used to show that the less stable hRPA:nt complex was quite compact, whereas the elongated hRPA3~~~ complex exists in both contracted and extended forms. Formation of the extended IiRPA3~~~ complex correlates with increased phosphorylation of the hRPA 29kl) subunit by DNA-dependent protein kinase.